The war between man and bacteria entered a new phase. With the rapid spread of antimicrobial resistant bacterial strains the currently used drug families start to fail and desperate calls for a new armory of agents are regularly voiced. Indeed, apart from the discovery of natural antibacterial peptides from plants and animals, there have been few new antibiotics developed in recent years. In this desperate situation the insect-derived cationic proline-rich antibacterial peptide family can come to the rescue. Unlike most other native antimicrobial peptides that interrupt the bacterial membrane permeability barrier, drosocin, pyrrhocoricin and apidaecin were shown to deactivate an intracellular bacteria-specific protein, and were shown to be completely non-toxic in vitro and in vivo to mammalian cells and live mice. [unreadable] [unreadable] We characterized the in vitro activity spectrum, in vivo efficacy and mode of action of these peptides, especially those of pyrrhocoricin, the most active member of this antimicrobial family against selected bacterial strains. Pyrrhocoricin and drosocin appear to kill bacteria by binding to the multihelical lid region of the 70 kDa heat shock protein DnaK and inhibit chaperone-assisted protein folding. Pyrrhocoricin and its designed analogs kill (-lactam-, tetracycline- and aminoglycoside-resistant Escherichia coli, Klebsiella pneumoniae, Salmonella typhimurium, and Haemophilus influenzae clinical isolates, among many others, in nanomolar or low micromolar concentrations. In addition, these peptides protect mice from experimental systemic E. coli or local H. influenzae lung infection when added intravenously (i.v.) or as an aerosol. Our multiply protected dimeric lead analogs retain all the high in vitro activities of the native compound, and show remarkable stabilities in mammalian sera. The activity spectrum of the pyrrhocoricin analogs, the urgent need for new antimicrobial compounds to treat human urinary tract infections (UTI) and the generally observable tissue distribution and renal clearance of peptidic drugs suggest that if appropriately developed, pyrrhocoricin derivatives will be suitable candidates for the treatment of UTI in the clinical setting. This grant application is concerned with the development of such a peptide-based drug candidate. [unreadable] [unreadable] In the Phase I stage of the studies, we will characterize the efficacy of the current dimeric lead pyrrhocoricin analog against a series of uropathogenic bacteria in vitro by using two different assay types. In addition, the in vivo potency, approximate dose and efficacious administration routes (i.v., subcutaneous or oral) will be characterized in a widely accepted mouse ascending UTI model, which uses an E. coli strain isolated from a human pyelonephritis case. If the lead compound shows promising in vitro and in vivo efficacy data, in Phase II, we will optimize the lead compound for further increased in vitro efficacy against uropathogenic bacterial strains, stability, oral bioavailability, lack of toxicity and reduced manufacturing costs. Four peptide analogs will be selected for further in vivo activity studies in mice. When all data are available, a final pyrrhocoricin derivative will be identified, and it will be investigated for its ability to treat experimental UTI in a rabbit model. In addition, the final drug candidate will be subjected to detailed toxicity studies in rats. [unreadable] [unreadable] Our hope is that by the end of the grant period, we will be able to provide a pyrrhocoricin-derived peptide for ensuing US Food and Drug Administration-approved preclinical and clinical evaluation against urinary tract infections. [unreadable] [unreadable]